RU2559080C1 - Method of producing of metal powders by hot spray - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to powder metallurgy. The spray of molten metal is dispersed by the surrounding concentric flow of the spraying gas with imposing of sound oscillations. Sound oscillations are created by means of at least two identical elastic rectangular plates located in a flow of spraying gas parallel to its axis and fixed on their width. Frequency of sound oscillations is determined by the known formula, then taking into account the obtained value, the elastic properties of material of plates and at the known length and width the thickness of plates from the known equation are determined.

EFFECT: increase of share of fine fraction in the pulverisate which is formed at molten metal dispersion is provided.

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Description

The invention relates to the field of powder metallurgy, in particular to methods for producing powders of aluminum, magnesium and their alloys by spraying molten metals with a gas stream.

A known method of spraying molten metals, including the dispersion of the molten metal by an external gas stream, concentric stream of the melt [1]. Known methods of spraying metal melts, providing increased dispersion of the obtained powder (pulverizate) by additional heating of the auxiliary gas [2], reducing the pressure in the spray chamber [3], additional dispersed introduction of hot gas into the spray zone [4] or into the metal wire [5], installation divider-destabilizers in the spray zone [6].

The closest in technical essence is the method of spraying liquid metals by dispersing a jet of melt surrounding it with a concentric gas stream with superimposed sound vibrations [7]. Sound vibrations with several discrete frequencies are generated by an annular resonance cavity located in the channel for supplying atomizing gas. The disadvantage of this method is the lack of correlation of the frequencies of the generated sound vibrations with the natural frequency of the oscillations of the melt jet during its interaction with the spraying gas.

The technical result of the invention is to increase the mass fraction of a finely divided fraction in the spray formed during the spraying of a molten metal.

The technical result is achieved by the fact that a method has been developed for producing metal powders by spraying melts, comprising dispersing a stream of metal melt with a concentric stream of atomizing gas surrounding it with the application of sound vibrations, characterized in that the sound vibrations are created by at least two identical elastic rectangular plates located in the spray flow gas parallel to its axis and fixed along their width, while the frequency of sound vibrations f is determined by the formula (kHz):

where u is the relative flow rate of the spraying gas and the jet of molten metal, m / s;

ρ is the density of the atomizing gas in the stream, kg / m ³ ;

σ is the surface tension coefficient of the molten metal, N / m;

and taking into account the obtained value of the frequency of sound vibrations f, the elastic properties of the material of the plates and for a given length and width, determine the thickness of the plates from the equation

where f is the frequency of sound vibrations, kHz;

E is the modulus of elasticity of the plate material, Pa;

ν is the Poisson's ratio of the plate material;

ρ _p is the density of the plate material, kg / m ³ ;

a - plate length, m;

b is the width of the plate, m;

h is the plate thickness, m

The resulting positive effect of the invention is associated with the following factors.

1. When the gas moves along a plate fixed from one edge, the plate begins to vibrate with a frequency equal to the frequency of its own vibrations. Oscillations of the plate, in turn, are transmitted to the gas stream flowing around it, which leads to non-stationary velocity fields in the gas stream. If the superimposed oscillation frequency of the gas stream coincides with the most unstable frequency of the liquid jet, then the amplitude of small perturbations on the jet surface rapidly increases (resonance phenomenon), which leads to the separation of small droplets from the jet surface and, therefore, improves the spray conditions.

2. The phenomenon of liquid spraying (destruction of its surface with the formation of a large number of small droplets) is associated with an increase in the amplitude and the appearance of instability of short waves on the surface of a liquid under the dynamic action of a gas stream. An analysis of the problem of the decay of a liquid jet by a high-speed blowing gas flow showed [8] that the increment of oscillations of the liquid surface has a maximum at the wave number

where u _g is the relative velocity of the gas and the jet at the surface of the liquid;

λ _max - wavelength of the most unstable oscillations.

From equation (1) follows the expression for the oscillation frequency of the most unstable short waves:

When the frequency of oscillations of the surface of the liquid f _{max is} achieved the maximum value of the increment of oscillations

where ρ _W is the density of the liquid.

, амплитуда колебаний ζ поверхности жидкости увеличивается в е раз, поскольку ζ~ехр(α·t).In time t equal to $$t_{\max }={\alpha }_{\max }^{-\mathrm{one}}$$

, the oscillation amplitude ζ of the liquid surface increases e times, since ζ ~ exp (α · t).

When the gas moves relative to the surface of the liquid, a turbulent boundary layer forms in the gas. The amplitude of the waves (roughnesses) on the liquid surface ζ and the gas velocity in the flow core and (equal to the gas velocity at the exit of the nozzle nozzle) are related to the gas velocity at the liquid surface by the ratio

where δ is the characteristic size of the liquid jet. The amplitude of the initial perturbations on the liquid surface usually does not exceed ζ = 10 ^-2 δ [8]; therefore, the gas velocity at the liquid surface is u _g = 0.217 u. Substituting this value in (1), we obtain the value of the frequency of oscillations that have a maximum perturbing effect on the liquid stream (melt)

3. When the gas moves along the plate, it begins to oscillate with its own frequency, determined by its size and physical properties of the material [9]:

- цилиндрическая жесткость пластины;Where

- cylindrical stiffness of the plate;

G _x , G _y , H _x , H _y , J _x , J _y are coefficients depending on the conditions of plate fixing and the vibration mode.

For the longitudinal vibrations of the plate clamped from one edge and the first mode, expression (5) is simplified (G _x = 0.597, Н _х = -0.087, G _y = Н _у = 0, J _x = 0.471, J _y = 12 / π ² ) and has the following form:

For practical calculations, formula (6) can be represented as

By choosing the plate material (E, ρ _p , ν) and its geometrical dimensions ( a , b, h), it is possible to achieve that the frequency range of the plate’s natural vibrations is in the frequency range close to the frequency of the maximum disturbing effect on the surface of the melt jet (4), thereby ensuring its effective destruction (dispersion).

4. The spray gas stream has a circular shape, therefore, for a uniform distribution of sound vibrations superimposed on the spray gas stream, the number of plates should be at least two, while the plates should be evenly spaced around the perimeter of the annular cavity and directed parallel to the axis of the spray gas stream. With a larger number of plates, the effectiveness of their impact on the gas stream and, therefore, on the melt stream increases.

The invention is illustrated by the following drawings.

FIG. 1. The scheme of the nozzle for spraying melts.

FIG. 2. The layout of the plates in the annular channel of the nozzle.

An example implementation of the method

In FIG. 1 shows an example implementation of the inventive method for producing metal powders by spraying melts. The nozzle for spraying melts consists of a housing 1, a cover 2, a nipple with a central channel for supplying the melt 3, a protective steel cover 4, a pipe 5 for supplying hot compressed gas and a pipe 6 for supplying the melt. An annular cavity 7 is made in the housing 1 for supplying compressed gas to the annular nozzle 8 formed by the outlet cones of the cap 2 and the nipple 3. In the annular cavity 7, plates 9 are mounted uniformly located over the cross section of the annular cavity (Fig. 2) and rigidly fixed from the inlet side part 10 of the annular cavity 7 (in Fig. 2 shows an embodiment of the nozzle with six plates). On the outer surface of the protective steel cover 4, an annular tide 11 is made, the height of which is not less than the width of the slit of the annular nozzle 8, contributing to the development of plate oscillations due to deviation of the gas flow.

The nozzle works as follows. Through the pipeline 5 through the inlet part 10 of the annular cavity 7, the gas enters the space between the plates 9. When the gas moves along the plates and flows around the annular tide 11, the plates begin to vibrate with their own frequency, determined by formula (7). The oscillations of the plates, in turn, are transmitted to the stream of atomizing gas flowing around them, which contributes to a more efficient dispersion of the melt.

We will evaluate the effectiveness of the claimed method on the example of obtaining aluminum powder according to the technology of LLC SUAL-PM [10]. To obtain pulverizate, the aluminum melt is sprayed with hot gas - nitrogen. Spraying is carried out by an ejection nozzle with a mass flow rate of aluminum melt of 0.04 kg / s through a nozzle with a diameter of 4 mm at a temperature of 900 ° C (σ = 0.84 N / m) and a mass flow rate of nitrogen of 0.2 kg / s at a temperature of 600 ° C and a pressure of 6 MPa. The spray gas is supplied through an annular nozzle with a slot width of 0.8 mm. The nozzle has an annular gas cavity with outer and inner diameters of 42 mm and 26 mm and a length of 40 mm.

For the indicated spraying conditions, the gas velocity at the nozzle nozzle exit is u = 550 m / s, the melt jet velocity is u _m = 1.3 m / s, the density of the spray gas in the stream is ρ = 0.4 kg / m ³ . The frequency value calculated by formula (4), which provides the maximum effect on the dispersion process, is f ~ 87 kHz.

Taking into account the dimensions of the annular cavity for supplying atomizing gas (Fig. 2), we choose the dimensions of the plates: b = 12 mm, a = 25 mm. As the material of the plates, 1Kh18N9T steel can be used (elastic modulus E = 200 GPa, density ρ _p = 7800 kg / m ³ , Poisson's ratio ν = 0.3) [11].

Substituting in the formula (7) the selected values of the plate dimensions ( a , b) and material characteristics (E = 200 GPa, ρ _p = 7800 kg / m ³ , ν = 0.3), we obtain the ratio for determining the plate thickness h providing the required frequency value natural vibrations f = 87 kHz. The calculated value of h = 1.92 mm.

Plates with the given characteristics create superimposed sound vibrations on the spray gas stream with a frequency that provides optimal conditions for spraying an aluminum jet.

Thus, the inventive method for producing metal powders by spraying melts increases the dynamic effect of the spraying gas stream on the melt stream due to the resonant amplification of oscillations of the liquid surface, which ensures the claimed positive effect — an increase in the mass fraction of the finely dispersed fraction in the atomizer formed when the metal melt is sprayed.

Literature

1. Fedorchenko IM, Andrievsky R. A. Fundamentals of powder metallurgy. - Kiev: Publishing House of the Academy of Sciences of the Ukrainian SSR, 1963. - 420 p.

2. Pat. RF 2022715, IPC B22F 9/08. A method of obtaining a highly dispersed spherical aluminum powder / V.N. Bunkov, V.A. Kondyrev, L.S. Golubtsov, N.T. Filimonov, V.A. Kovalev. - No. 4936976/02; declared 05/16/1991; publ. 11/15/1994.

3. Pat. RF 2026157, IPC B22F 9/08. A method of producing aluminum powder / V.N. Bunkov, V.A. Kondyrev, N.T. Filimonov, V.A. Kovalev, L.S. Golubtsov. - No. 4841131/02; declared 06/19/1990; publ. 01/09/1995.

4. Pat. RF 2296648, IPC B22F 9/08. Nozzle for spraying molten metals / A.V. Kuksa, A.V. Molkov, A.V. Gubanov. - No. 2005132356/02; declared 10/19/2005; publ. 04/10/2007.

5. Pat. RF 2283728, IPC B22F 9/08. Nozzle for spraying molten metals / A.V. Kuksa, A.V. Molkov, M.P. Kononov, A.V. Gubanov, S.V. Linkov. - No. 2005105853; declared 03/02/2005; publ. 09/20/2006.

6. Pat. RF 2321475, IPC B22F 9/08. Nozzle for spraying molten metals / A.V. Kuksa, A.V. Molkov, A.V. Gubanov, S.V. Linkov. - No. 2006115192/02; declared 05/02/2006; publ. 04/10/2008.

7. Patent US No. 4640806, IPC B22F 9/08. Process for atomizing liquid metals to produce finely granular powder / Thomas Duerig, Marcel Escudier, Jakob Keller, Killwangen. - declared. 10/01/1985; publ. 02/03/1987.

8. Levich V.G. Physicochemical hydrodynamics. - M .: Fizmatgiz, 1950 .-- 699 p.

9. Gontkevich B.C. Natural vibrations of plates and shells. - Kiev: Naukova Dumka, 1964 .-- 278 p.

10. Technological instructions for the production of spherical dispersed, finely dispersed and with titanium and silicon additives spray guns by the spraying of molten aluminum in branch No. 2 of the company SUAL-PM LLC. - TI 48-0106-36-1-10, Shelekhov, 2010.

11. Directory of machine builder in 6 vols. T. 1-6. Under. ed. Acherkana N.S. - L .: Mashgiz, 1960 .-- 740 p.

Claims (1)

A method for producing metal powders by spraying melts, comprising dispersing a stream of metal melt with a concentric stream of atomizing gas surrounding it with superimposed sound vibrations, characterized in that the sound vibrations are generated by at least two identical elastic rectangular plates located in the stream of spray gas parallel to its axis and fixed along their width, while the frequency of sound vibrations f is determined by the formula (kHz):

,
where u is the relative flow rate of the spraying gas and the jet of molten metal, m / s;
ρ is the density of the atomizing gas in the stream, kg / m ³ ;
σ is the surface tension coefficient of the molten metal, N / m;
and taking into account the obtained value of the frequency of sound vibrations f, the elastic properties of the material of the plates and for a given length and width, determine the thickness of the plates from the equation

where f is the frequency of sound vibrations, kHz;
E is the modulus of elasticity of the plate material, Pa;
ν is the Poisson's ratio of the plate material;
ρ _p is the density of the plate material, kg / m ³ ;
a - plate length, m;
b is the width of the plate, m;
h is the plate thickness, m

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